Perhaps at night when it's quiet and it wouldn't congest the frequency, a "call rolling" might be handy to give the ATCO a chance to look up and see that all is as should be. By the way, there's no need to have all four ATCOs on duty at that time of the night. They need to be rested for the morning rush which begins only a few hours later. This incident has NOTHING to do with the number of ATCOs in the tower at the time.